Assertion (A): The descending limb of Henle's loop contain hypertonic urine and ascending Henle's loop urine becomes hypotonic.
Reason (R): The descending Henle's loop is impermeable to Na+ and ascending Henle's loop is impermeable to water.
1. Both (A) and (R) are true and (R) is the correct explanation of (A).
2. Both (A) and (R) are true but (R) is not the correct explanation of (A).
3. (A) is true but (R) is false.
4. Both (A) and (R) are false.